Led device having improved contrast

ABSTRACT

A light-emitting diode device, including a substrate; and a reflective electrode and a semi-transparent electrode formed over the substrate and an unpatterned white light-emitting layer formed between the reflective electrode and the semi-transparent electrode, the reflective electrode, semi-transparent electrode, and unpatterned white-light-emitting layer forming an optical microcavity, and wherein either the reflective or semi-transparent electrodes is patterned to form a plurality of independently controllable light-emitting elements with at least one light-emitting element having no color filter. Color filters are formed over a side of the semi-transparent electrodes opposite the unpatterned white light-emitting layer in correspondence with the light-emitting elements, the color filters having at least two different colors. Additionally, a reflected-light absorbing layer is located over all of the light-emitting elements.

FIELD OF THE INVENTION

The present invention relates to light-emitting diode (LED) devices, andmore particularly, to LED device structures for improving light outputand ambient contrast.

BACKGROUND OF THE INVENTION

Emissive flat-panel display devices are widely used in conjunction withcomputing devices and in particular with portable devices. Thesedisplays are often used in public areas with significant ambientillumination.

Organic light emitting diodes (OLED) have many advantages in aflat-panel display device and are useful in optical systems. U.S. Pat.No. 6,384,529 issued May 7, 2002 to Tang et al. shows an OLED colordisplay that includes an array of OLED light emitting elements (pixels).Light is emitted from a pixel when a current is passed through anorganic material, the frequency of the light depending on the nature ofthe organic material that is used. The organic materials are placed upona substrate between electrodes, with an encapsulating cover layer orplate. In such a display, light can be emitted through the substrate (abottom emitter) or through the encapsulating cover (a top emitter), orboth. The emitted light is Lambertian, which is it is emitted equally inevery direction. Because OLED devices typically employ a reflective backelectrode, ambient light is typically reflected and the contrast of thedisplay is of great concern. Similarly, inorganic LED devices, forexample comprising quantum dots in a polycrystalline semiconductormatrix, located between a reflective electrode and a transparent orsemi-transparent electrode, suffer from ambient contrast concerns.

The ambient contrast ratio of a display is a performance factor thatrequires high-light emission combined with a low reflectivity. BecauseLED devices typically employ a reflective back electrode, providing agood ambient contrast ratio can be problematic. It is known to use acircular polarizer affixed to the surface of the display so that lightincident on the display is absorbed by the polarizer, while lightemitted by the display is not. For example, WO0210845 A2 entitled, “HighDurability Circular Polarizer for use with Emissive Displays” publishedFeb. 7, 2002 describes a high durability circular polarizer including anunprotected K-type polarizer and a quarter-wavelength retarder. Thiscircular polarizer is designed for use with an emissive display modulesuch as an organic light emitting diode or a plasma display device.However, while effective in reducing reflections in high-ambient lightconditions, for example, outdoors on sunny days, such polarizers do notalways provide adequate contrast and they absorb more than half of theemitted light.

Other methods of improving contrast and readability in a display rely onincreased light output. For example, microcavity structures are known toincrease the light emitted from an OLED device structure. Such opticalcavity structures are also known as microcavities or opticalmicrocavities (and are used interchangeably herein) and rely on apatterned deposition of different color light-emitting organic materialsover a substrate between a reflective electrode and a semi-transparentelectrode. Light emitters having different colors are formed within anoptical cavity tuned to a desired peak wavelength of light correspondingto the color of light emitted by the patterned organic materials. U.S.Pat. No. 6,680,570 describes an organic light-emitting device withimproved color control employing spacer layers to form an opticalcavity. FIG. 7 illustrates such a prior-art, active-matrix,bottom-emitting microcavity device employing a substrate 10 withactive-matrix thin-film components 30, planarization structures 32 and34, and a semitransparent electrode 12. Patterned organic materials 14R,14G, and 14B corresponding to red, green, and blue light emission aredeposited in a light-emitting layer 14. Optical spacers 26R, 26G, and26B are employed to form optical cavities 60, 62, and 64 tuned to thedesired peak wavelengths of red, green, and blue light, respectively toemit red light 80, green light 82, and blue light 84. A cover 20 can beemployed to protect and encapsulate the device. While such designs areuseful, they require a patterned organic material deposition technologythat is difficult to scale to large substrates. Moreover, microcavitydevices typically suffer from an unacceptable angular dependence oncolor. It is also known to employ a color filter with a microcavitystructure, for example as taught in U.S. Pat. No. 7,189,238. However,while useful, such an approach does not improve the manufacturability ofthe device and provides inadequate contrast ratio under someillumination conditions.

U.S. Pat. No. 5,554,911 entitled “Light-emitting elements” describes amulti-color light-emitting element having at least two opticalmicrocavity structures with respectively different optical lengthsdetermining their emission wavelengths. Each microcavity structureincludes an organic material as a light-emitting region, which may be asingle film of uniform thickness in the element. U.S. Pat. No. 6,861,800entitled “Tuned microcavity color OLED display” describes a microcavityOLED device having an array of pixels divided into at least twodifferent color pixel sets, each color pixel set emitting a differentpredetermined color light over a common substrate, wherein each pixel inthe array includes a metallic bottom-electrode layer disposed over thesubstrate and a metallic electrode layer spaced from the metallicbottom-electrode layer. The material for the semitransparent metallicelectrode layer includes Ag, Au, or alloys thereof. The thickness of thesemitransparent metallic electrode layer, the combined thickness of theorganic layers and the transparent conductive phase-layer, and also theplacement of the light-emitting layer are selected so that each pixel inthe display forms a tuned microcavity OLED device having an emissionoutput efficiency above that of a comparable OLED device without themicrocavity. U.S. Pat. No. 5,949,187 describes an OLED with a firstmicrocavity including a first transparent spacer and a first mirrorstack positioned on the first spacer to reflect light back into the OLEDand to define an optical length of the first microcavity. The opticallength of the first microcavity being such that light emitted from thefirst microcavity has a first spectrum. A second microcavity includes asecond transparent spacer positioned adjacent the first microcavity anda second mirror stack positioned on the second spacer reflects lighttoward the first microcavity and defines an optical length of the secondmicrocavity. The optical length of the second microcavity being suchthat light emitted from the second microcavity has a second spectrum.Additional microcavities can be placed in the structure to furtherenhance and alter the light spectrum. Such designs, however, may haveincreased manufacturing costs, lower light output than desired, andreflectance larger than may be desired, as well as significant colorchange with changes in viewing angle.

US20060066228 A1 entitled, “Reducing or eliminating color change formicrocavity OLED devices” by Antoniadis, discloses a microcavity OLEDdevice that minimizes or eliminates color change at different viewingangles. The OLED device can be, for example, an OLED display or an OLEDlight source used for area illumination. This OLED device includes amulti-layer mirror on a substrate, and each of the layers is comprisedof a non-absorbing material. The OLED device also includes a firstelectrode on the multi-layered first mirror, and the first electrode issubstantially transparent. An emissive layer is on the first electrode.A second electrode is on the emissive layer, and the second electrode issubstantially reflective and functions as a mirror. The multi-layermirror and the second electrode form a microcavity. On a front surfaceof the substrate is a light modulation thin film. The light modulationthin film can be any one of: a cut-off color filter, a band-pass colorfilter, a brightness enhancing film, a microstructure that attenuates anemission spectrum at an angle at which there is a perceived colorchange, or a microstructure that redistributes wavelengths so theoutputted emission spectrums have the same perceived color. Again suchdesigns may have increased manufacturing costs and reflectance largerthan may be desired.

U.S. Pat. No. 7,030,553 entitled, “OLED device having microcavity gamutsub-pixels and a within gamut sub-pixel” by Winters et al discloses anexample of a prior-art microcavity device. This disclosure describes anOLED device including an array of light emitting pixels, each pixelincluding sub-pixels having organic layers including at least oneemissive layer that produces light and spaced electrodes. There are atleast three gamut sub-pixels that produce colors that define a colorgamut and at least one sub-pixel that produces light within the colorgamut produced by the gamut sub-pixels. At least one of the gamutsub-pixels includes a reflector and a semitransparent reflector whichfunction to form a microcavity. However, this design employs a patternedsemi-transparent electrode that can be difficult to manufacture in atop-emitting format. Moreover, applicants have determined that colorpurity may be reduced in such a design and reflectance will likely belarger than desired.

One approach to overcoming material deposition problems on largesubstrates is to employ a single emissive layer, for example awhite-light emitter, together with color filters for forming afull-color display, as is taught in U.S. Pat. No. 6,987,355 entitled“Stacked OLED Display having Improved Efficiency” by Cok and commonlyassigned herewith. However, the use of color filters substantiallyreduces the efficiency of the device.

There still remains a need, therefore, for an improved light-emittingstructure that overcomes shortcomings in the prior art and thatincreases the light output and ambient contrast ratio of an LED device.

SUMMARY OF THE INVENTION

The present invention is directed to a light-emitting diode device thatincludes a substrate. A reflective electrode and a semi-transparentelectrode are formed over the substrate and an unpatterned whitelight-emitting layer is formed between the reflective electrode and thesemi-transparent electrode. Accordingly, the reflective electrode,semi-transparent electrode, and unpatterned white-light-emitting layer,form an optical microcavity. Either the reflective or semi-transparentelectrode is patterned to form independently controllable light-emittingelements with at least one light-emitting element having no colorfilter. Additionally, a plurality of color filters is formed over a sideof the semi-transparent electrodes opposite the white light-emittinglayer in correspondence with the light-emitting elements, the colorfilters having at least two different colors. Finally, a reflected-lightabsorbing layer is located over all of the light-emitting elements.

ADVANTAGES

The present invention has the advantage that it increases the lightoutput and ambient contrast of an LED device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial cross section of a top-emitter LED deviceaccording to an embodiment of the present invention;

FIG. 2 illustrates a partial cross section of a top-emitter LED deviceaccording to another embodiment of the present invention;

FIG. 3 illustrates optical spacers useful with various embodiments ofthe present invention;

FIG. 4 illustrates optical spacers and a light scattering layer usefulwith various embodiments of the present invention;

FIG. 5 illustrates optical spacers useful with another embodiment of thepresent invention;

FIG. 6 illustrates a partial cross section of a bottom-emitter LEDdevice according to an embodiment of the present invention;

FIG. 7 illustrates a partial cross section of a prior-art bottom-emitterLED device;

FIG. 8 is a flow-chart illustrating a method of the present invention;

FIG. 9 is a system employing an LED device according to the presentinvention; and

FIG. 10 is a partial cross-section of an LED device exemplaryillustrating absorption of ambient light according to an embodiment ofthe present invention.

It will be understood that the figures are not to scale since theindividual layers are too thin and the thickness differences of variouslayers too great to permit depiction to scale.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a light-emitting diode device according to anembodiment of the present invention comprises a substrate 10, areflective electrode 12, and a semi-transparent electrode 16 formed overthe substrate 10. Either the reflective electrodes 12 orsemi-transparent electrodes 16 is patterned to form a plurality ofindependently controllable light-emitting elements 50, 52, 54, and 56.The independently controllable light-emitting elements are controlled,for example by thin-film electronic components 30 formed on thesubstrate 10. The other electrode may be unpatterned and electricallycommon to all of the light-emitting elements. An unpatterned whitelight-emitting layer 14 is formed between the reflective electrode 12and the semi-transparent electrode 16. Reflective electrode 12,semi-transparent electrode 16, and unpatterned white-light-emittinglayer 14 form optical microcavities 60, 62, 64, and 66. A plurality ofcolor filters 40R, 40G, and 40B are formed over a side of thesemi-transparent electrode 16 opposite the white light-emitting layer 14in correspondence with the light-emitting elements 50, 52, 54 with atleast one light-emitting element 56 having no color filter, the colorfilters 40R, 40G, and 40B having at least two different colors, and areflected-light absorbing layer located over all of the light-emittingelements 18. A black matrix 40K can be employed to absorb ambient lightbetween the light-emitting elements 50, 52, 54, 56. Planarizing andinsulating layers 32 and 34 are provided to electrically separate theindependently controllable light-emitting elements.

As employed in the present invention, a pixel is a multi-color pictureelement comprising three or more sub-pixels, each sub-pixel includes anindependently controlled light emitter emitting light of a differentcolor. Typically, pixels include red, green, and blue sub-pixels (an RGBconfiguration). In addition, as employed in this disclosure, a whitesub-pixel is also included in each pixel (an RGBW configuration). When awhite sub-pixel is employed in an RGBW configuration, if the whitesub-pixel has a greater luminous efficacy than any of the red, green, orblue sub-pixels (as will generally be true due to the lack of a colorfilter over the white sub-pixel) increased brightness or reduced powerutilization is obtained for images containing regions havinglow-to-moderate color saturation (i.e. having a significant graycomponent). The light-emitting elements 50, 52, 54, 56 correspond tosub-pixels.

The reflected-light absorbing layer 18 preferentially absorbs ambientlight that passes through it and that is reflected from the reflectiveelectrode 12, and passes through the reflected-light absorbing layer 18again (as is shown in FIG. 10 and discussed in more detail below). Theambient light is preferentially absorbed relative to the emitted lightfrom the unpatterned light-emissive layer 14 (even if the light emittedfrom the unpatterned light-emissive layer 14 is reflected from thereflective electrode 12). For example, in one embodiment of the presentinvention, the reflected-light absorbing layer 18 may be a circularpolarizer. Alternatively, a neutral density filter may be employed.

The optical microcavities 60, 62, and 64 are typically tuned topreferentially emit light with the same peak frequency as thecorresponding color filters 40R, 40G, and 40B, thus providing thedesired color saturation and gamut of the device. The opticalmicrocavity 66 can be tuned to preferentially emit light correspondingto a peak emission of the white light emitter 56. The optical cavityassociated with the white light-emitting element 56 may be tuned tooptimize the color(s) of light emitted by the unpatterned light-emittinglayer 14, for example to meet a desired white point. That is, theoptical cavity of each light-emitting element having a color filter istuned to emit light at a peak wavelength approximately corresponding tothe peak transmission wavelength of the color filter. The white lightemitted by the unpatterned light-emitting layer 14 may not be on thePlanckian locus, but is preferably at least a broadband light.

As shown in FIG. 1, the present invention may employ spacer layers 26R,26G, 26B, and 26W having different thicknesses between the reflectiveelectrode 12 and the light-emissive layer 14. The different thicknessesare chosen to tune the optical response of the different opticalcavities 60, 62, 64. In an alternative embodiment, shown in FIG. 3, theoptical cavities may be tuned by employing transparent spacer layers13R, 13G, 13B, and 13W between a reflective layer 11 and a transparentconductive layer 15, the reflective layer 11 and transparent conductivelayer 15 comprising the reflective electrode 12. In other embodiments ofthe present invention (not shown) spacer layers may be located in otherpositions, for example, between the light-emissive layer 14 and thesemi-transparent electrode 16.

Note that, since an optical microcavity structure tends to produce sharpresonance peaks in the spectrum, it is difficult to produce thebroadband emission necessary for the white sub-pixel. Hence, in FIG. 1,it is preferable to have the spacer layer 26W of sufficient thicknessthat the optical microcavity produces multiple resonance peaks withinthe visible spectrum in order to produce a good white color. In order toavoid the need for such a thick spacer layer and to produce a whitesub-pixel likely having higher efficiency, we now discuss otherexemplary embodiments.

Referring to another embodiment in FIG. 2, a light-scattering layer 22of light-scattering particles 24 scatters light emitted by thelight-emitting element 18 that has no color filter (i.e., the Wsub-pixel) light-scattering layer 22 is located on or near a high-indexlayer of the sub-pixel to mix together the different colored light thatwould, in the absence of layer 22, be emitted at different angles(including some at angles beyond the critical angle for total internalreflection, hence light would not be able to escape the device), therebyproducing a broadband spectrum even with a relatively thin spacing layer26W. By frustrating total internal reflection, light-scattering layer 22can also increase the total amount of emitted light. According to someembodiments of the present invention, the light-scattering layer 22 oflight-scattering particles 24 can be located over the semi-transparentelectrode 16 on a side of the semi-transparent electrode 16 opposite thewhite light-emitting layer 14 (as shown in FIG. 2) or between areflective layer 11 and conductive transparent layer 15 comprisingelectrode 12 (as shown in FIG. 4).

Referring to FIG. 5 in yet another exemplary embodiment, the opticalcavity of one or more of the light-emitting elements that have no colorfilter comprises a plurality of optical microcavities 27R, 27G, 27B,each optical microcavity tuned to emit light at a different wavelength.The different wavelengths may, in combination, be perceived as white.For example, complementary colors of light can be employed, for example,blue and yellow or red and cyan light. In alternative embodiments, theoptical microcavities are tuned to primary colors of light, for examplered, green, and blue. In yet other alternatives, more colors can beemployed, for example, red, orange, yellow, green, cyan, and blue lightthat together are perceived as white.

In yet another alternative embodiment, the optical cavity length of theoptical microcavity of one or more of the light-emitting elements thathave no color filter may vary continuously, rather than in steps. Such acontinuous change in the optical cavity length of the opticalmicrocavity is considered to define a plurality of opticalmicrocavities, each optical microcavity tuned to emit light at adifferent wavelength.

The light-emitting layer may comprise organic materials, for exampleOLED or PLED materials. Alternatively, the light-emitting layer cancomprise inorganic materials, for example, quantum dots formed in apoly-crystalline semiconductor matrix. The light-emitting layer emits aspectrum having peaks, for example having two or more peaks, wherein thetwo or more peaks correspond to the desired colors of emitted light. Thefrequencies of the peaks are of complementary colors or of primarycolors as noted above.

In one embodiment, the present invention is a top-emitter device, asshown in FIG. 1. In an alternative embodiment, the device is abottom-emitter device, as shown in FIG. 6.

Current, for example as supplied through thin-film transistors, passesthrough the light-emitting layer, causing light to be emitted. Some ofthe emitted light passes directly out of the device or through the colorfilters and out of the device. Other light is reflected from thereflective electrode 16 and passes out of the device. Other light,emitted at a higher angle to the normal, is trapped via total internalreflection. The microcavity structures serve to reduce the angle ofemission of the emitted light, thereby, reducing the amount of trappedlight and also focusing more of the desired light in the forwarddirection. If a scattering layer is present, the scattering layer servesto scatter out the trapped light, although the optical effectiveness ofthe microcavity may then be reduced. Emitted light passes through thecircular polarizer once and light that is not polarized in the directionof the circular polarizer is absorbed, reducing the amount of emittedlight.

Referring to FIG. 10, ambient light 70 incident on the device passesfirst through the circular polarizer 18 and at least half of the ambientlight is absorbed. A portion of the remaining ambient light 71 is thenabsorbed by the color filters 40 (for the color sub-pixels) and then afurther portion 72 is absorbed by the microcavity structures. That lightnot absorbed by the microcavity structures and reflected from the backelectrode 12 then passes into the color filters 40 (where present) and afurther portion of light 73 is absorbed. The circular polarizer thenabsorbs almost all of the remaining, reflected light 74 so that theexternally reflected ambient light 75 is greatly reduced as describedfurther below.

Referring to FIG. 8, an LED device can be formed using the followingsteps: step 100: forming a substrate, step 105: forming a reflectiveelectrode and a semi-transparent electrode over the substrate, step 110:forming an unpatterned white light-emitting layer between the reflectiveelectrode and the semi-transparent electrode, wherein the reflectiveelectrode, semi-transparent electrode, and unpatternedwhite-light-emitting layer form an optical microcavity, and wherein atleast one of the reflective or semi-transparent electrodes is patternedto form a plurality of independently controllable light-emittingelements with at least one light-emitting element having no colorfilter, step 115: forming a plurality of color filters over a side ofthe semi-transparent electrodes opposite the white light-emitting layerin correspondence with the light-emitting elements, the color filtershaving at least two different colors, and step 120: forming areflected-light absorbing layer over all of the light-emitting elements.

In a patterned device, different materials are employed to emit light ofdifferent colors in response to a current. In contrast, in anunpatterned device, the same materials are employed to emit a singlecolor, for example, white; and the light emitted by color sub-pixels iscolored by employing color filters in combination with the white-lightemitter. Often, a white-light emitter will include a combination ofmaterials in one or more unpatterned layers that each emit a differentcolor, for example, blue and yellow, to emit a light that is perceived,overall, to be white. The important point is that however many organicmaterials are included in a single layer, or however many layers areincluded, the layers are unpatterned and their aggregate emissionemployed in all of the sub-pixels in all of the pixels.

It is known in the prior art that, in LED devices, light may be trappedby total internal reflection in the high-optical-index layers thatactually emit light, or high-optical index charge-control layers, orhigh-optical index transparent electrodes. Light emitted at low anglesto the normal may be emitted from the device while light emitted at arelatively higher angle to the normal may be trapped in thehigh-optical-index layers. By employing a microcavity structure, theemission of light at high angles is reduced so that more light isemitted from the device at relatively lower angles to the normal. In analternative technical approach, light-scattering materials defeat totalinternal reflection and may also increase the amount of light emittedfrom such a device.

However, it is also true that the color of light emitted frommicrocavity structures has a dependence on the viewing angle. Thisangular dependence can be extremely irritating to viewers, in particularfor applications in which a large viewing angle is valued. In contrast,as demonstrated by applicant, a light-scattering layer has no suchdependence. This color shift with angle is especially noticeable forcolor sub-pixels using a white-light emitter. However, the color filtersemployed in the present invention not only absorb ambient light, theyalso reduce the observed dependence on angle of the light color that onenormally has with a microcavity device.

The white-light sub-pixel will also shift in color. However, because thewhite-light sub-pixel of FIG. 1 typically employs a multi-peakwhite-light emitter, the shift in color will largely be a shift in whitepoint rather than a shift to a specific color. While this shift maychange the observed white point of the device, when combined with arelatively minor color shift in the color sub-pixels, or embodiment ofthe present invention having a white-point shift provides improved imagequality. When the viewing angle is increased, the light emissiontypically has a shorter wavelength. However, when at least twocomplementary colors are emitted from the white-light-subpixel, theaverage white color emitted by the two or more colors together remainsrelatively constant compared to the changes in color of the individualcolor.

When the configuration of FIG. 2 is employed, the color of white-lightemission does not change with angle and a good white point for thedevice is maintained when the device is viewed at angles other than thenormal. When the configuration of FIG. 3, is employed, the multiplecavities employed in the white sub-pixel has an even greater effect thanthat of FIG. 1, in that the light emitted by the multiple peaks mayshift together, but the perceived color shift is reduced and only anunobjectionable, minor white-point shift may be observed.

However, the color shift reduction found with increasing angle for thecolor sub-pixels provided by the present invention does reduce theluminance of these color sub-pixels. Such a reduction in luminance isless noticeable and objectionable to viewers than a shift in color.Moreover, to the extent that the color sub-pixels decrease in luminance,while the luminance of the white sub-pixels is constant (although awhite-point shift may occur) as a result of changing viewing angles, thenet effect may be a reduction in overall color saturation. Such a colorsaturation reduction may be negligible for some images (i.e. thoseimages with little saturated color) and less noticeable than a change incolor for those images with strongly saturated colors. Hence, improvedimage quality may be obtained. Moreover, since most images arerelatively unsaturated, the net effect is often relatively minor.

The ambient contrast ratio of an LED device is a critical factor in itsperformance, in particular for outdoor applications where ambientillumination can be extremely high. Ambient contrast is defined as aratio of the light emitted by the device and the ambient light reflectedfrom the device. An improved performance is obtained when light emissionis increased and light reflection is decreased. According to the presentinvention, an improvement in LED device performance is found byincreasing the light output by employing a microcavity structure and awhite sub-pixel, while ambient light reflection is decreased byemploying color filters for the color sub-pixels and a microcavityabsorber for all of the sub-pixels, together with a circular polarizer.Moreover, by employing an unpatterned white-light emitter, manufacturingcosts are decreased for large-size glass substrates for large-sizedisplay devices. In particular, the present invention may be employedfor display devices, especially large-format display devices, forexample, large-screen televisions or monitors.

Applicants have constructed numerous OLED devices employingmicrocavities, both with patterned, colored emitters and with whiteemitters and have studied their performance together with theperformance of circular polarizers and color filters. Moreover, opticalmodeling tools have been employed to understand the performance of thepresent invention under a variety of circumstances. In general, awhite-light-emitting, unpatterned OLED device, employing a microcavityand color filters, can be expected to roughly double the light output ofthe colored pixels in comparison to a white-light-emitting, unpatternedOLED device without a microcavity and color filters. The colorsub-pixels will increase in light output the most, while thewhite-light-emitting sub-pixel will only change in light output by amultiplicative factor of roughly 0.6 to 1.2, since it is more difficultto increase broadband light output than narrow-band light in amicrocavity structure. However, since the white-light-emitting sub-pixelis more efficient (by about a factor of three) than the color sub-pixels(because no color filters are employed in the white sub-pixel), the useof a white sub-pixel improves the overall performance of an OLED deviceas most images have few saturated colors and the more-efficientwhite-light emitter is used disproportionately. Applicants havedemonstrated that the use of such a white sub-pixel in a color filterdesign can improve the overall display's device performance by a factorof two for imaging applications.

An explicit optical modeling calculation of a device employing atwo-peak OLED emitter was performed with a reflective aluminum backelectrode 12 and a thin silver semi-transparent electrode 16. With acommercially-available color filter set, the red light output on-axisincreased by 2.8 times, the green light output is increased by 2.0times, and the blue light output is increased by 1.75 times and improvedcolor gamut is obtained. With a proprietary color filter set developedby applicants that allows for improved color gamut at the cost ofsomewhat decreased efficiency, the factor by which the on-axis lightoutput is increased when the microcavity structure of the presentinvention is employed is further increased by 5-15% for the red, blue,and green sub-pixels. Again, improved color gamut is also improved dueto the microcavity. For the white-light-emitting sub-pixel, theperformance of the device of FIG. 1 typically has a lower performance(for example, 0.6 times the output relative to a prior-art device notemploying the microcavity structure) compared to the devices shown inFIG. 2 (for example, 1.0 to 1.3 times) and FIG. 3 (for example, 1.0-1.2times).

By employing an Ag reflective back electrode, the device performanceimproves further by 5-20%. Thus, if the proprietary color filters areemployed together with the Ag reflective back electrode, the red lightoutput may increase by approximately 3.5 times, the green light outputmay increase by approximately 2.4 times, and the blue light output mayincrease by approximately 2.1 times, while the white light output mayincrease by approximately 0.7 to 1.6 times, depending on theconfiguration.

According to the present invention, ambient light is absorbed by thecircular polarizer (for all sub-pixels), the color filters (for thecolor sub-pixels), and by the microcavities (for all sub-pixels). Notethat the color filter thickness is effectively doubled for the ambientlight relative to the emitted light since the ambient light reflectedfrom the back electrode of the color sub-pixels passes through the colorfilters twice. A dual-peak white-light emitter in a microcavitystructure will also absorb a significant amount of ambient light (forexample half), further reducing reflectivity. In the case of FIG. 2,employing light-scattering particles in the white sub-pixels willdecrease the effectiveness of the circular polarizer somewhat, asdemonstrated by applicant, for example providing a reflection of 8%, butat the same time can increase light output from the white sub-pixel byapproximately a factor of two. This arrangement can provide improvedperformance in some ambient illumination conditions and for someapplications. The example of FIG. 3 can further increase deviceperformance by more effectively emitting light while still doing anexcellent job of absorbing ambient light.

Applicants have measured the performance of various attributes of thepresent invention and modeled the integrated performance of suchdevices. The performance of various embodiments of the present inventioncompared to the prior art is summarized in the table below. In thistable, the “relative radiance” describes the amount of light emittedon-axis (and is estimated in the case of the structures of FIGS. 1, 2,and 5 by taking an equally-weighted average of the gain factor for thered, green, blue, and white sub-pixels), the “RGB color” refers to thepurity of the color sub-pixels, the “white color” refers to the accuracyof the white (i.e. how close the white light is to a standard whitepoint such as D65), the “reflectance” is a measure of the ambient lightreflected from the device, and the “angular color white” refers to thechange in color of the white-light emission when viewed at angles otherthan the normal angle. “WRGBW” refers to an unpatternedwhite-light-emitting LED material with red, green, and blue colorsub-pixels, and a white-light sub-pixel. “WRGB” refers to an unpatternedwhite-light-emitting LED material with red, green, and blue colorsub-pixels only. Sub-pixels are all assumed to be the same size, usedequally, and the comparison is made to the prior-art configuration witha relative radiance of 1.0 using the best estimate above. A circularpolarizer having a single-pass transmission of 44% and allowing 1.6% ofambient light that is incident on a device with an ideal rear reflectorto be reflected from the device is used. The proprietary filtersreferenced above are used for the color filters. No black matrix isemployed but, if present, would further decrease the quantity ofreflected light.

Relative RGB White Overall Angular Color Structure Radiance Color ColorReflectance White WRGBW - No Microcavity 1.0 Good Excellent   34%Moderate No Circular Polarizer (Prior art) WRGBW - No Microcavity 0.44Good Excellent  0.5% Moderate with Circular Polarizer (Prior art FIG. 7)WRGB - Microcavity 0.44 Excellent 0.06% With Circular Polarizer WRGBW -Microcavity ~2.2 Excellent Poor   16% Poor No Circular Polarizer WRGBW -Microcavity ~1.0 Excellent Poor 0.25% Poor With Circular Polarizer(FIG. 1) WRGBW - Microcavity ~1.0 Excellent Excellent  0.7% ExcellentCircular Polarizer, and Scattering Layer (FIG. 2) WRGBW - Microcavity,~1.0 Excellent Excellent 0.25% Moderate Circular Polarizer, Whitemicrocavity sub elements (FIG. 5)

As can be seen from the table, the present invention, as embodied inFIGS. 1, 2, and 3, provide an excellent combination of useful features.For the embodiments of FIGS. 1, 2, and 5, the additional light emittedby the microcavity is absorbed by the circular polarizer. In the absenceof the circular polarizer, the light output by the embodiments of FIGS.1, 2, and 5 will increase from a factor of approximately 1.0 to a factorof approximately 2.27 with a consequent increase in reflectivity.

Applicants have also modeled the change in color due to angle ofemission for the white sub-pixel of FIG. 5. By employing an overallchange in optical cavity thickness of 160 nm for the plurality ofoptical microcavities in the white sub-pixel, the shift in color due toangle can be reduced to that seen in non-optical-microcavity devices.

Referring to FIG. 9, the present invention may be employed in a displaysystem comprising an LED device 200 (shown in greater detail in FIGS.1-6, and discussed thoroughly above) and a controller 210 that drivesthe LED device 200 in response to a display signal 220.

LED devices of this invention can employ various well-known opticaleffects in order to enhance their properties if desired. This includesoptimizing layer thicknesses to yield maximum light transmission,providing dielectric mirror structures, providing anti-glare oranti-reflection coatings over the display, providing neutral density, orproviding color conversion filters over the display. Filters,polarizers, and anti-glare or anti-reflection coatings can bespecifically provided over or as part of the cover or substrate.

The present invention can be practiced with either active- orpassive-matrix OLED devices, and is particularly useful in displaydevices. In a preferred embodiment, the present invention is employed ina flat-panel OLED device composed of small molecule or polymeric OLEDsas disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep.6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29,1991 to VanSlyke et al. Inorganic devices, for example employing quantumdots formed in a polycrystalline semiconductor matrix (for example, astaught in 11/622,266 by Kahen), and employing organic or inorganiccharge-control layers, or hybrid organic/inorganic devices can beemployed. Many combinations and variations of organic or inorganiclight-emitting displays can be used to fabricate such a device,including both active- and passive-matrix displays having either a top-or bottom-emitter architecture.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 substrate-   11 reflective layer-   12 reflective electrode-   13, 13R, 13G, 13B, 13W spacer-   14 light-emitting layer(s)-   15 transparent conductive layer-   16 semi-transparent electrode-   18 reflected-light absorbing layer-   20 cover-   22 light-scattering layer-   24 light-scattering particles-   26R, 26G, 26B, 26W spacer-   27R, 27G, 27B spacer-   30 thin-film circuitry-   32 insulator-   34 insulator-   40, 40R, 40G, 40B color filter-   40K black matrix-   50, 52, 54, 56 light-emitting elements-   60, 62, 64, 66 optical microcavity-   70, 71, 72, 73, 74, 75 light rays-   80, 82, 84 light-   100 form substrate step-   105 form reflective electrode step-   110 form light-emitting layer step-   115 form color filters step-   120 form reflected-light-absorbing layer step-   200 device-   210 controller-   220 signal

1. A light-emitting diode device, comprising: a) a substrate; b) areflective electrode and a semi-transparent electrode formed over thesubstrate and an unpatterned white light-emitting layer formed betweenthe reflective electrode and the semi-transparent electrode, thereflective electrode, semi-transparent electrode, and unpatternedwhite-light-emitting layer forming an optical microcavity, and whereineither the reflective or semi-transparent electrode is patterned to forma plurality of independently controllable light-emitting elements; c) aplurality of color filters formed over a side of the semi-transparentelectrodes opposite the unpatterned white light-emitting layer incorrespondence with the light-emitting elements, the plurality of colorfilters having at least two different colors and wherein at least onelight-emitting element has no color filter; and d) a reflected-lightabsorbing layer located over all of the light-emitting elements.
 2. Thelight-emitting diode device of claim 1 wherein the reflected-lightabsorbing layer is a circular polarizer.
 3. The light-emitting diodedevice of claim 1 wherein the device is a top-emitter device.
 4. Thelight-emitting diode device of claim 1 wherein the device is abottom-emitter device.
 5. The light-emitting diode device of claim 1wherein the optical microcavity of each light-emitting element having acolor filter is tuned to emit light at a peak wavelength approximatelycorresponding to the peak transmission wavelength of the color filter.6. The light-emitting diode device of claim 1 further comprising alight-scattering layer that scatters light emitted by the at least onelight-emitting element that has no color filter.
 7. The light-emittingdiode device of claim 6 wherein the reflective electrode comprises areflective layer and a transparent conductive layer, and thelight-scattering layer is formed between the reflective and transparentconductive layers.
 8. The light-emitting diode device of claim 6 whereinthe light-scattering layer is formed on a side of the semi-transparentelectrode opposite the white light-emitting layer.
 9. The light-emittingdiode device of claim 1 wherein the at least one light-emitting elementthat has no color filter comprises a plurality of optical microcavities,each optical microcavity tuned to emit light at a different wavelength.10. The light-emitting diode device of claim 9 wherein the opticalmicrocavities of the at least one light-emitting element that has nocolor filter are tuned to emit colors that, in combination, areperceived as white.
 11. The light-emitting diode device of claim 9wherein the optical microcavities of the at least one light-emittingelement that has no color filter are tuned to emit complementary colorsof light.
 12. The light-emitting diode device of claim 11 wherein theoptical microcavities of the at least one light-emitting element thathas no color filter are tuned to emit blue and yellow light or red andcyan light.
 13. The light-emitting diode device of claim 9 wherein theoptical microcavities are tuned to emit primary colors of light.
 14. Thelight-emitting diode device of claim 9 wherein the optical microcavitiesare tuned to emit red, green, yellow, cyan, or blue light.
 15. Thelight-emitting diode device of claim 1, wherein the white light-emittinglayer comprises organic materials.
 16. The light-emitting diode deviceof claim 1 wherein the white light-emitting layer comprises inorganicquantum dots formed in a poly-crystalline semiconductor matrix.
 17. Amethod of making the LED device, comprising the steps of: a) forming asubstrate; b) forming a reflective electrode and a semi-transparentelectrode over the substrate; c) forming an unpatterned whitelight-emitting layer between the reflective electrode and thesemi-transparent electrode; wherein the reflective electrode,semi-transparent electrode, and unpatterned white-light-emitting layerform an optical microcavity, and wherein either the reflective orsemi-transparent electrodes is patterned to form a plurality ofindependently controllable light-emitting elements; d) forming aplurality of color filters over a side of the semi-transparent electrodeopposite the unpatterned white light-emitting layer in correspondencewith the light-emitting elements, the plurality of color filters havingat least two different colors and wherein at least one light-emittingelement has no color filter; and e) forming a reflected-light absorbinglayer over all of the light-emitting elements.
 18. A display systemcomprising: an LED device, including: a) a substrate; b) a reflectiveelectrode and a semi-transparent electrode formed over the substrate anda unpatterned white light-emitting layer formed between the reflectiveelectrode and the semi-transparent electrode, the reflective electrode,semi-transparent electrode, and unpatterned white-light-emitting layerforming an optical microcavity, and wherein either the reflective orsemi-transparent electrodes is patterned to form a plurality ofindependently controllable light-emitting elements; c) a plurality ofcolor filters formed over a side of the semi-transparent electrodeopposite the unpatterned white light-emitting layer in correspondencewith the light-emitting elements, the plurality of color filters havingat least two different colors and wherein at least one light-emittingelement has no color filter; d) a reflected-light absorbing layerlocated over all of the light-emitting elements; and e) a controllerthat drives the LED device in response to a display signal.
 19. Thelight-emitting diode device of claim 1 wherein the white light-emittinglayer emits light having a spectrum with two or more peaks, wherein atleast one of the peaks has a peak frequency corresponding to thefrequency at which the microcavity is tuned.